Polymers and copolymers of conjugated dienes such as polybutadiene, polyisoprene and styrene-butadiene rubbers possess physical properties which make them suitable for many important applications such as synthetic rubbers and as additives to other polymeric systems such as, for example, high impact polystyrene (HIPS). Moreover, HIPS can be manufactured by polymerization of styrene in the presence of 5-10% dissolved polybutadiene or butadiene copolymer rubber. Early in the polymerization, phase separation begins because of the immiscibility of the rubber within the polystyrene being formed and the depletion of the styrene phase. Grafting of polybutadiene with the polystyrene then takes place. Toughness, as well as other mechanical and rheological properties of HIPS, is strongly affected by the nature of the rubber phase. In this regard, some of the characteristics of the rubber which may be modified to control the overall HIPS performance include concentration, volume, particle size, grafting and cross-linking ability, molecular weight, and viscosity.
One focus of the present invention is use of polybutadiene as an additive in HIPS or ABS resins. Specifically, the present invention addresses the desire that the polybutadiene additive have useful molecular weight and viscosity ranges. In this regard, strictly linear polybutadiene of low molecular weight typically has a low Mooney viscosity, making the polybutadiene difficult to handle, while a tetra-coupled version of the same low molecular weight polymer is too high to be processed. One mechanism to achieve a desired molecular weight and viscosity is to use a blend of tetra-coupled and linear polymer chains.
One method for the manufacture of copolymers having linear and branched segments rubbery composition includes a blend of from 40-94 parts by weight (pbw) Component A and from 60-66 pbw of Component B. Component A includes a rubbery (co)polymer(s) of conjugated dienes, and at least 60% by weight of the components in the A portion are branched polymers. Component B is generally the same as Component A, but consists of linear (co)polymer(s). The process of manufacture involves forming Component A in a first step, Component B in a second step and performing a third step of blending A and B.
Another process is directed to polymerizing at least one diene monomer to a conversion between 30 and 70% to produce low molecular polydiene chains; joining from 20 to 70% of those chains with a suitable branching agent; and allowing the polymerization to continue to produce a polydiene rubber blend. However, by failing to perform sufficient conversion in the first step, insufficient solution viscosity is produced.